Climate and soil micro‐organisms drive soil phosphorus fractions in coastal dune systems

The importance of soil phosphorus (P) is likely to increase in coming decades due to the growing atmospheric nitrogen (N) deposition originated by industrial and agricultural activities. We currently lack a proper understanding of the main drivers of soil P pools in coastal dunes, which rank among the most valued priority conservation areas worldwide. Here, we evaluated the joint effects of biotic (i.e. microbial abundance and richness, vegetation and cryptogams cover) and abiotic (i.e. pH and aridity) factors on labile, medium‐lability and recalcitrant soil P pools across a wide aridity gradient in the Atlantic coast of the Iberian Peninsula. Climate determined the availability of medium‐lability, recalcitrant and total P, but had a minor net effect on labile P, which was positively and significantly related to the presence of plants, mosses and lichens. Medium‐lability P was significantly influenced by soil bacterial richness and abundance (positively and negatively, respectively). Our results suggest that micro‐organisms transfer P from medium‐lability pool to more labile one. At the same time, increases in bacterial richness associated to biofilms might be involved in the thickening of the medium‐lability P pool in our climosequence. These bacterial‐mediated transfers would confer resistance to the labile P pool under future climate change and uncover an important role of soil micro‐organisms as modulators of the geochemical P cycle. Resumen Es probable que la importancia del fósforo (P) del suelo aumente en las próximas décadas debido a la creciente deposición atmosférica de nitrógeno (N) originada por las actividades agrícolas e industriales. Actualmente no tenemos un conocimiento adecuado de los principales factores que controlan las fracciones de P del suelo en dunas costeras, las cuales se encuentran entre las áreas de conservación prioritaria más valoradas en todo el mundo. Aquí, evaluamos los efectos conjuntos tanto de factores bióticos (es decir, abundancia y riqueza de microorganismos y cobertura de vegetación y criptógamas) como abióticos (es decir, pH y aridez) sobre las fracciones de P lábil, de labilidad media y recalcitrante a lo largo de un amplio gradiente de aridez en la costa atlántica de la Península Ibérica. El clima determinó la disponibilidad del P de labilidad media, recalcitrante y total, pero tuvo un efecto neto menor en el P lábil, el cual fue positivamente relacionado con la presencia de plantas, musgos y líquenes. El P de la